1. Field of the Invention
The present invention relates to a method of recovering a solvent by a heat pump system. More particularly, it relates to a method of recovering a solvent which method will be useful in industrial fields requiring the recovery and re-utilization of volatile solutes and volatile solvents present in a mother liquor, in a system in which non-volatile solids or liquids also are dissolved in the mother liquor.
2. Description of the Related Prior Art
In the fields of industrial chemistry, petrochemistry, pharmaceuticals, foodstuffs, etc., volatile solutes and volatile solvents in a mother liquor have been recovered and re-utilized, in many cases from a system in which non-volatile solids or liquids also are dissolved in the mother liquor. Various methods of recovering the solvents have been proposed in the past as described below.
(1) Vapor re-compression system distillation method:
This system is referred to as a "heat pump distillation system" and has already been put into practical use in the fields of petrochemistry and foodstuffs as an energy-saving, economical distillation method for reducing the heat input required for effecting distillation (e.g., Japanese Patent Laid-Open No. 209602/1982).
(2) Evaporation technique:
This technique has been put into practical use as a technique for vaporizing and separating a solvent when non-volatile solids or liquids are dissolved in a volatile liquid. A large number of evaporators have been proposed.
(3) Multiple effect evaporation technique:
When the concentration of the solutes is low and large quantities of heat input are necessary in order to vaporize and separate the solvent, this technique sequentially connects a plurality of evaporators and sequentially causes evaporation in the evaporators by the use of the vapor generated by the evaporators of the preceding stages. This technique has already gained a wide application as a technique for reducing the heat input required in industrial fields.
(4) Continuous rectification technique not having recovery section:
This technique is referred to as a "still head" which has a rectifier having a fractionating column at a still section, and has conventionally been used as a batch-wise or continuous rectification technique.
Among them, the vapor re-compression system distillation method and the multiple effect evaporation technique have been used primarily, in view of the energy savings they provide.
However, when applied to the solvent recovery method which is the object of the present invention, the conventional vapor re-compression system distillation method and the multiple effect evaporation technique are not free from the following problems. According to the conventional vapor re-compression system (heat pump system) distillation method, the concentrating (rectifying) section and the recovery (stripping) section of the distilling column are necessary in order to recover the column top fraction and the column bottom fraction, respectively. Hence, the mother liquor (feed material) must be fed to an intermediate location between the concentrating section and the recovery section. When the mother liquor contains a non-volatile solid and liquid, therefore, the non-volatile solid and liquid must flow down through the packing materials and trays of the recovery section, and this results in contamination and clogging of the column and eventual failure of the operation.
The multiple effect evaporation technique can process the mother liquor containing the non-volatile solids and liquids and can reduce the necessary heat input, but it cannot withdraw products having a specified, high purity. The combination technique with the "still head" can be employed to withdraw distillate products having a high purity, but a large number of evaporators equipped with the "still head" are necessary in order to withdraw distillate and bottom products having high purities, while reducing the heat input. In other words, in order to reduce the necessary heat input, the multiple effect evaporation technique needs a large number of effect columns and presents the problem that the cost of the installation is increased.